At level 64, shaman get the delicious wrath of air totem. This stick of goodness increases spell damage and healing by 101. When my elemental shaman got this baby at 64, he had only a couple of hundred spell damage; this totem alone bumped it by a mega 50%. As I'm pushing 1000 spell damage at the moment wrath of air would give me a respectable 10% increase. Our raid healers would be lucky to get a 5 or 6% increase to healing.
At level 70, a talented prayer of fortitude will increase your stamina by 102, conferring 1020 hit points. To a mage with 7000 health, that's a very sweet improvement of nearly 15%; to a warrior with 13,000 health, it's less than 8%.
Do you see where I'm going with this? There are a plethora of buffs out there that just don't scale very well for end-game content. In general, the better your gear, the less useful the buff. Just ask a raiding druid what they think of mark of the wild...
This isn't the case with everything. A beastmaster hunter's ferocious inspiration, while only a temporary buff, increases all damage by 3% for its duration. Compare that to a marksman hunter's trueshot aura, which provides a flat increase to attack power alone.
I wonder if buffs should more consistently scale. What would be the harm of using percentages instead of flat numbers?
I suppose you could argue it doesn't make any difference. Blizzard has tuned instances to the capabilities of the classes as they are now. If buffs were to get more powerful over time, instead of less, they'd simply retune the content to arrive at the same balance.
But it does seem more in keeping with the idea of end-game progression that characters should get more powerful, not less, over time.
22 hours ago